Add fuzzing tests

src/fuzzing.rs

@@ -0,0 +1,8 @@
+/// Fuzzing target for simplify().
+///
+/// Calls simplify() and panics if simplify() panics.
+/// Does not return any vaule to avoid exposing internal crate types.
+#[cfg(fuzzing)]
+pub fn simplify(mut input: String, is_chat_message: bool) {
+    crate::simplify::simplify(input, is_chat_message);
+}